Use new `proxy_enable` and `proxy_url` settings

[link2xt]

With core release 1.143.0 old socks5_* settings are deprecated and proxy_* should be used instead.

This should be done when done is updated from 1.142.x.

https://github.com/deltachat/deltachat-android/pull/3292 gives an idea how the screen looks like in Android

Step 1

• [x] Change Setting Types: socks5_enabled, socks5_host, socks5_port, socks5_user, socks5_password => proxy_enabled, proxy_url
• [x] Adapt form: One text field for one proxy_url (placeholder: "Enter proxy URL here"
• [x] UI: Show explaining text: "Supported proxy types: HTTP(S), SOCKS5 and Shadowsocks"

Step 2 (in separate PR)

• [ ] UI: add client side validation
• [ ] UI: Show connectivity status & proxy type
• [ ] UI: enable multiple proxy url entries
• [ ] if a proxy is clicked/selected/added, enable "use proxy" toggle

Step 3 (in separate PR)

• [ ] Proxy UI should also be moved out of configuration and into a separate screen like done on Android: https://github.com/deltachat/deltachat-android/pull/3277

Sharing is probably not necessary on desktop

[nicodh]

@link2xt : do we not support username & password for proxies anymore or can it be entered in the form username:password@some.host:8080

[link2xt]

Username and password are supported in the form of socks5://username:password@host:port. Username and password should be URL-encoded.